5. HOW IMPORTANT ARE INTERACTIONS?

The interacting galaxies of Arp's 1966
catalogue of peculiar galaxies or
Arp and Madore's
1987 catalogue of southern peculiar galaxies are
convincingly, some of the most impressive objects in the sky, both from a
scientific as well as an aesthetic point of view. Galaxy interactions
cover a wide range of interaction strength, including minor disturbances
due to relatively distant flybys or the presence of smaller companions,
strong distortions (e.g. tidal bridges and tails) due to close encounters,
larger systems cannibalizing smaller ones, or mergers of systems of similar
mass. Interactions have also been implicated to explain the progression
of
activity in AGN, from the most distant energetic quasars, through radio
galaxies and seyferts, to nearby systems with relatively little nuclear
activity, i.e. AGN power was enhanced in the past when the galaxian
environment was richer. That interactions are indeed taking place is
firmly established, both from an observational as well as a theoretical
point of view. What is less clear is the frequency with which these events
are occurring now and in the past, whether interactions must be factored
into a scheme of ``normal'' galaxy evolution, or indeed whether they might
even be the dominant driver of evolution. Put another way, to what
extent are galaxian properties governed by initial conditions (nature), and
to what extent are they driven by environmental effects (nurture)?

A well-established observational effect related to the nature/nurture
question is the morphology-density (M-D) relation. The M-D
relation states that, both within and outside of clusters of galaxies, the
fractional abundance of elliptical and S0 galaxies with respect to spirals
increases with increasing space density of galaxies. Also, a giant cD
galaxy is sometimes present at the dynamical center of some clusters.
The M-D relation, in some form or other, has been known for at least a
century (see
Roberts and Haynes
1994), but has been quantified more recently
over a wide range of density regimes by
Dressler 1980 and by
Postman and Geller
1984.

Possible explanations for the M-D relation have been summarized by
Whitmore 1990.
``Nature'' explanations include a) angular momentum in the protocloud,
i.e. the higher angular momentum material in the outer parts of the cloud
will have a slower rate of star formation and therefore form disk galaxies
after the initial spheroidal component is formed towards the cluster
center, and b) strength of the initial density enhancement, i.e. denser
material towards the center of the protocluster would form stars at a
faster rate during collapse, resulting in more elliptical galaxies towards
the cluster center. The ``nurture'', possibilities include a) tidal
``shaking'', in which the internal distributions of galaxies passing through
the higher density cluster core are shaken and rearranged resulting in an
earlier type galaxy, b) tidal stripping, in which interactions induce
stripping of the gas and stars which then fall towards the cluster center,
possibly building up a central cD galaxy there, c) ram pressure stripping
and gas evaporation, whereby galaxies passing near the center of a cluster
are swept by the denser intracluster gas, quenching star formation and
producing more S0s at the expense of normal spirals, d) merging of disk
galaxies to form ellipticals in the cluster center or e) cannibalism by
the central cD galaxy of the numerous galaxies around it. There are
various nature/nurture hybrid models as well. The M-D example is
presented, not so as to discuss the pros and cons of the various models,
but to illustrate that various processes can lead to the same observational
result. Unless we can identify specific signatures which can differentiate
between these processes, the nature/nurture conundrum will continue to
challenge us. For more discussion, see papers in
Thuan et al.
1992.

Some of the more interesting work in recent years has focused on
environmental effects (nurture) in shaping the morphology of galaxies. In
particular, it is becoming apparent that strong interactions and mergers
can, in a sense, create galaxies, either by tidally tearing off
pieces of the victim to form independent dwarf galaxies, or by tidally
randomizing the stellar fields of two merging disk galaxies to create a
giant elliptical.

The idea that a tidal remnant or clump resulting from an interaction might
eventually develop into an independent dwarf galaxy was originally
suggested by
Zwicky (1956) and
has enjoyed renewed attention lately due to
the increasing sophistication of computers and N-body codes.
Barnes and Hernquist
1992b, for example, have demonstrated that self-gravitating
``clumps'' with masses up to several x 108M can form in
tidal tails. The theoretical results have received observational support
from the detection of massive optical clumps of up to 8 x 109M
(e.g. in NGC 4038 / 9,
Mirabel et al.
1992) and also of
neutral hydrogen clumps of about 108M (e.g. IC 2163 / NGC 2207,
Elmegreen et
al. 1993), which have recently been discovered
in tidal tails. Mirabel et al. have shown that the integrated properties
of the optical clumps resemble those of dwarf irregular galaxies.

More dramatically, the evidence is mounting that two disk galaxies can
merge to produce an elliptical galaxy. Theoretical simulations have shown
that elliptical-like remnants result from disk-disk collisions and,
observationally, galaxies in various stages of merging also suggest this
(see sequence of illustrations in
Barnes and Hernquist
1992a). A good
example of a late stage merger which is well on its way to becoming a
``classical'' elliptical is the so-called ``atoms for peace'' galaxy,
NGC 7252.
This galaxy resembles an elliptical at its center in short
exposures, but displays highly disturbed morphology and tidal tails in the
outer regions which are visible in deeper exposures
(Schweizer 1982).
Other evidence that this is an elliptical in the making includes, a)
chaotic motions observed in the main body, b) shells and ripples similar
to those observed in other ellipticals, c) the azimuthally averaged radial
brightness profile described by an R(1/4) law, d) the
galaxy falling
within the scatter of the Faber-Jackson relation but outside of the scatter
of the
Tully-Fisher relation, and e) mechanisms which appear to be at work to rid
the optical galaxy of its neutral gas
(Hibbard et al.
1994). N-body
simulations have also successfully reproduced the morphology and velocity
field of NGC 7252
as the product of two colliding disk galaxies
(Borne and Richstone
1991;
Mihos et al.
1993).

Over the past 10 years, the evidence in favour of ellipticals forming from
merging spirals has been increasing (e.g. see
Schweizer
1986). Some have
also argued (e.g. Efstathiou 1990) that few galaxies were not shaped in
some way by galaxy interactions. There have been some lingering problems
with this scenario, however. For example, in the merging scenario, one
would expect the specific frequency of globular clusters in both spirals
and ellipticals to be similar, whereas in fact, the specific frequency in
ellipticals is sometimes much higher (e.g. M87). The fact that there are
as many red (and therefore, presumably, early type) field galaxies
at redshifts of about 1 as there are today (Lilly, private communication)
tends to argue against substantial
merger-driven evolution over this time frame. Also, the systematic trend
for early type galaxies in rich clusters to be bluer with increasing
redshift (e.g.
Aragón-Salamanca
et al. 1992) argues for a
single burst
of star formation at high z, followed by steady, passive evolution.
Some of these problems seem less compelling, however, in the light of new
HST results which appear to suggest the formation of globular clusters
during encounters (e.g. NGC 1275) and that spectacular galaxy-galaxy
interactions are occuring, even at redshifts of 1. Therefore,
the evidence continues to mount that interactions can be significant
drivers of galaxy formation and evolution. Whether interactions are the
dominant drivers overall and at what redshifts they are most
important are still open questions. For further discussion of the
importance of interactions, see
Ellis (1993).

The above considerations assume, of course, that we know which galaxies are
interacting and which are not. Various indicators have been used in the
past to identify the interacting galaxies, virtually always based on some
optical criteria, for example, the existence of tails and/or bridges, or
other ``disturbed'' or ``peculiar'' morphology. Typically, catalogues such
as the Atlas and Catalogue of Interacting Galaxies
(Verontsov-Velyaminov
1959,
1977), or the Atlas of Peculiar Galaxies
(Arp 1966) are
used, and/or samples gleaned from visual inspection. Such
studies will only select the strongest, most obvious interactions. In
other attempts to include weaker interactions, some investigators have
studied samples of galaxies with specific peculiar properties not based on
optical morphology, like nuclear starbursts or Seyfert activity, and
searched for correlations with the presence of companions. For example,
there is good evidence that nuclear starbursts are fuelled by infalling gas
from the surrounding interstellar medium as a result of an external tidal
disturbance. For reviews of interaction-driven nuclear activity, see
Heckman (1990) or
papers in Shlosman
(1994).

The question could be raised, then, as to whether an interaction might be
strong enough to significantly affect the evolutionary history of the
galaxy, yet not so strong as to produce obvious morphological
peculiarities. As an example, I refer again to the NGC 5775 / 4 pair. By
virtue of their apparent proximity, it might be supposed that these
galaxies would be interacting, and it is now clear from HI observations
(see Fig. 4) that this is indeed the
case (Irwin
1994). However, no
obvious optical distortions are visible in either galaxy (but note the very
faint optical bridges between the galaxies on the Palomar Observatory Sky
Survey blue print), no particularly strong nuclear activity (starburst or
AGN) is present, and the system had not previously been identified as
``interacting'' in the literature. Yet a significant amount of gas
(possibly up to 10 M yr-1, though this is rather uncertain)
may be transferring onto NGC 5775 from its companion. Over a typical
interaction timescale, this is certainly sufficient to affect the star
formation, and hence the evolution, of both galaxies. Enhanced
overall star formation at the present epoch is indeed implied from the high
IR luminosity of NGC 5775.

What is the best way of confirming that an interaction might be taking
place if the optical morphologies are relatively normal? One possibility
is IR brightness, as implied above. For example, enhanced IR luminosity
is known to be correlated with a high incidence of optical distortions (and
by implication with interactions) in more distant IRAS galaxies
(Rowan-Robinson
1991 and references therein). Also, enhanced nuclear IR
emission might be expected in interactions (even those which do not result
in gas transfer between galaxies) because of the starbursting which can be
triggered there. However, given that there may be reasons other than
interactions as to why IR emission is enhanced (for example, star formation
induced by nuclear outflow, a contribution to IR luminosity from sources
not related to star formation, or a higher abundance of dust in the ISM),
some other direct evidence, such as the HI connection between NGC 5775 /
4 will probably be required. Note that neutral
hydrogen imaging, though time
consuming, is probably the best way to directly confirm that an interaction
is occurring. Not only is the necessary velocity information obtained, but
this component appears to be the most sensitive tracer of bridges, tails,
or other tidally induced distortions. This is because the gaseous
component can diverge from the stellar component due to shocks (see
Barnes and Hernquist
1992a) and also because the original HI distribution in the
galaxy is the most extensive to begin with (see Section 3.1). For example, for
NGC 5775 / 4, the ratio of emission in a typical intergalactic
region to the peak brightness of NGC 5775 is about 1/1000 in the optical,
1/100 for the radio continuum, and 1/10 for HI. Other good
examples of interactions which are most obvious in HI are the Leo Triplet
(see Haynes et
al. 1984) and the M82 group (see
Scoville et
al. 1994).